Studies on mutagenesis focus on the UmuDC proteins in E. coli or homologues (e.g., MucAB) in plasmids. A chemiluminescent assay has been used to analyze the processing of UmuD to a mutagenically active form (UmuD'). In E. coli, this process is inefficient, requiring derepressed levels of activated RecA for UmuD cleavage. We have detected a UmuD-like protein in diverse enterobacteria. Several non-mutable strains were defective in UmuD processing, while the poorly mutable S. typhimurium, was, surprisingly, most efficient. To study MucA cleavage, we overproduced the MucA protein using the lambdaPL promoter. In vitro cleavage was substantially faster than that of the functionally homologous UmuD. To study the interactions between proteins involved in mutagenic DNA repair, we used a UmuD' protein affinity column. Using a specialized tranducing lambda phage, the umuDC operon was deleted and replaced with the cat gene. The delta(umuDC)595::cat mutation was subsequently transferred into a variety of genetic backgrounds. We found that the UmuDC proteins, normally required for inducible mutagenesis, are not essential for cell survival. The processes by which DNA lesions are repaired in mammalian cells were studied with various in vivo or in vitro systems. In primate cells, after UV exposure, DNA repair functions are inhibited at early times (due to consumption of repair factors) and then enhanced (due to de novo synthesis of these factors). This modulation of repair is correlated with the level of a novel damage-specific DNA-binding (DDB) protein. DDB is likely to have an important role in the repair of UV-damaged DNA on the basis of its high affinity for UV-damaged DNA in vitro, enhancement by UV exposure of cells in vivo, and absence in some patients with the UV-repair deficiency, xeroderma pigmentosum (group E). DDB may be the prototype of a family of mammalian excision repair proteins. The function of the SV40 protein small t antigen (tag) is not well understood. It is known that tag is required for the efficient large T antigen-mediated induction of cellular DNA replication and transformation when nondividing cells are infected in vitro by SV40, but tag is not required in proliferating cells. We also find that SV40 mutants lacking tag transform rapidly proliferating cell types in vivo, but not cells with a low proliferative rate. We have now found a biochemical function for tag: its ability to inhibit SV40 DNA replication in an in vitro system. The tag protein, by inhibiting the cellular phosphatase PP2A, may alter the phosphorylation state of large T antigen required for the initiation of viral replication. We are pursuing the possibility that tag also inhibits antioncogene activity, explaining its requirement in non-dividing cells.